This invention relates to an interface apparatus having a circuit emulation function. More particularly, the invention relates to an interface apparatus, which is provided between a digital transmission network, such as an STS-N (where N represents an integer) digital transmission network, and an ATM network, for eliminating an unnecessary part of frame data (e.g., STS-3 data) in the digital transmission network, forming only the necessary part of the data into cells, sending the cells to the ATM network, assembling frame data (e.g., STS-3 data) of the digital transmission network from cells received from the ATM network, and sending the assembled frame data to the digital transmission network.
As shown in FIG. 30, a dedicated service network constituted by an optical transmission line of a SONET (Synchronous Optical Network) enables communication by connecting terminals CPE such as DS3, STS-3 or STS-12 terminals by optical transmission lines via multiplexer/demultiplexers MDX and digital cross-connect systems DCCS. Dedicated service networks differ from ordinary telephone communication networks or the like in that they do not require call-connect and call-disconnect processing and in that entrances to and exists from such networks are decided in a semi-permanent manner. A DCCS has functions such as path switching, multiplexing/demultiplexing and frame add/drop but does not have a function for dynamic routing as does an exchange. For this reason, conventional dedicated service networks have problems relating to network maintenance. For example, it is not easy to carry out network rerouting and expansion (1) when it is desired to establish an alternative path between terminals owing to a decline in the quality of or the occurrence of a failure in a specific transmission path and (2) when it is desired to expand a network owing to an increase in traffic over a specific path.
Accordingly, there is demand for a dedicated service network in which network rerouting and expansion can be performed by substituting an ATM switch for a DCCS and issuing commands and making settings directly from a center. FIG. 31 is a conceptual view for a case where an OC3 DCCS is replaced by an ATM switch. In order to replace the OC3 DCCS by the ATM switch, not only are a switch ATM-SW and a controller CNTL necessary but it is also required that the transmission paths have interfaces (STS-3 CES) INF1 to INF4 through which the optical transmission lines of an OC3 SONET interwork with the ATM switch. Each of the interfaces INF1 to INF4 has a circuit emulation (CE) function. The interfaces INF1, INF2 convert STS-3 frame data, which has been received from an optical transmission line, to ATM cells, and the interfaces INF3, INF4 assemble ATM cells into an STS-3 frame format and send the frames to an optical transmission line.
In a case where STS-N (where N represents an integer) frame data is converted to ATM cells in such an interface, it is necessary to convert the frame data and send it to an ATM network in such a manner that the transmission band is reduced. Moreover, it is necessary for data in the original STS-N frame format to be assembled from received cells and transmitted to a digital transmission line.
An interface for receiving cells from an ATM network, assembling the cells into a frame format and transmitting the results to an optical transmission line is provided with a receive buffer of a prescribed capacity for two purposes, namely (1) for clock transfer and (2) to accommodate fluctuations in cell arrival. An initial fill level (IFL) is set for the receive buffer in such a manner that (1) the receive buffer will not be emptied if the ATM cell arrival interval is such that cells do not arrive over an assumed period of time and (2) the receive buffer will not overflow if the ATM cell arrival interval is such that too many cells arrive over an assumed period of time. If the IFL fluctuates so as to cause starvation or overflow of the receive buffer, the continuity of information cannot be maintained and cell discard occurs. Accordingly, it is necessary to exercise control in such a manner that the IFL will not fluctuate even if cell loss, insertion of erroneous cells or garbling of cells, etc., occur, thereby assuring that the receive buffer will not experience starvation or overflow.
Accordingly, an object of the present invention is to arrange it so that STS-N frame data can be converted to ATM cells so as to reduce the transmission band, and so that the STS-N frame format can be assembled from received cells.
Another object of the present invention is to arrange it so that a large quantity of reproduced data will not be lost if P-format cells are lost, and so that an IFL that has been set for a receive buffer will not fluctuate owing to cell loss, insertion of erroneous cells or garbling of cells, etc., thereby assuring that the receive buffer will not experience starvation or overflow.
According to the present invention, (1) the part of a frame that contains a payload and overhead data, which indicates the starting position of a low-bit-rate data block multiplexed into a frame, is defined as a cell conversion zone; (2) the data in the cell conversion zone is converted cells when a prescribed position in the cell conversion zone is adopted as a reference position; and (3) a pointer that specifies the reference position is included in a prescribed cell. If this arrangement is adopted, only the portion of the overhead that will be necessary later need be converted cells. As a result, the amount of transmitted data can be reduced, thereby making it possible to reduce the transmission band. Since the pointer that specifies the prescribed position (e.g., the starting position) of the cell conversion zone in a frame is incorporated in a cell, the interface on the receiving side can assemble a frame from received cells upon referring to the pointer. A cell that contains a pointer shall be referred to as a P-format cell and a cell that does not contain a pointer shall be referred to as a non-P-format cell.
Further, according to the present invention, a cell payload of AAL Type 1 received from an ATM network is stored in a receive buffer in sync with the clock of the ATM network, and the cell payload is read out of the receive buffer in sync with a clock on the side of a digital transmission line, thereby performing clock transfer. Further, a pointer is detected from a cell payload that has been read out of the receive buffer, the starting position of a cell conversion zone is identified based upon the pointer, and a frame is assembled using data output from the receive buffer based upon the starting position of the cell conversion zone. If this arrangement is adopted, the interface can recognize the cell conversion zone in a frame and the starting position of each byte in the frame based upon the pointer, as a result of which it is possible to assemble the original frame from the data read out of the receive buffer. In case of AAL Type 1, one cycle is constructed by eight cells and a sequence count SC (=0 to 7) is assigned to each cell.
Further, according to the present invention, timing at which a succeeding pointer will appear is predicted by pointer detection. If a P-format cell does not appear at the predicted timing and the cell at this timing is an invalid cell or dummy cell, the cell is judged to be a P-format cell. Further, if an arriving cell is a cell whose sequence count SC is 6, this cell is an invalid cell or dummy cell and, moreover, a P-format cell has not yet been detected between SC=0 and SC=7, then the arriving cell is judged to be a P-format cell.
If this arrangement is adopted, a P-format cell can be generated even if a P-format cell is lost owing to cell loss or cell garbling, and it can be so arranged that the IFL that has been set for a receive buffer will not fluctuate. This makes it possible to assure that starvation and overflow will not occur.
Further, timing at which a succeeding pointer will appear is predicted by pointer detection. When a P-format cell is lost at the predicted timing, a prescribed cell is judged to be a P-format cell based upon this timing. This makes it possible to raise the precision with which P-format cells are judged. By virtue of the foregoing, a cell can be assumed to be a P-format cell correctly every cycle (SC=0xcx9c7) and the IFL of a receive buffer can be prevented from fluctuating as a result of judging a P-format cell to be a non-P-format cell or judging a non-P-format cell to be a P-format cell. Moreover, even if a P-format cell is lost, the fact that a prescribed cell is assumed to be a P-format cell assures that a large quantity of reproduced data will not be lost as a consequence of loss of P-format cells.
Further, according to the present invention, a cell (dummy cell or invalid cell) that has a high likelihood of being a P-format cell is assumed to be a P-format cell when a P-format cell has been lost. As a result, a cell can be assumed to be a P-format cell correctly every cycle (SC=0xcx9c7) and therefore the IFL of a receive buffer can be prevented from fluctuating. Moreover, even if a P-format cell is lost, the fact that a prescribed cell is assumed to be a P-format cell assures that a large quantity of reproduced data will not be lost as a consequence of loss of P-format cells.
Further, according to the present invention, whether a cell having an even-numbered SC value is a P-format cell is checked successively every cycle and, when a P-format cell is lost, a cell for which SC is equal to 6 is assumed to be a P-format cell. If this expedient is adopted, a cell can be assumed to be a P-format cell through a simple arrangement and it possible to assure agreement between the bands of STS-3 and ATM networks.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.